1. Field of the Invention
This invention pertains to the analysis of well data for the purpose of predicting the probability of a drill string becoming stuck. More particularly, the invention pertains to a method of modeling the probability of a drill string becoming stuck in a well under given drilling conditions as determined from observations in previously drilled wells and of applying that model to determine the probability of sticking in a well being drilled.
2. Description of the Prior Art
It is well known to those in the art of oil well drilling that it is not unusual for a drill string to become stuck in the well bore and that this sticking can result from a wide variety of causes. Typically, these causes are grouped into those that cause mechanical sticking and those that cause differential sticking. Mechanical sticking, for instance, can be caused by key seating, undergauge hole, well bore instability, poor hole cleaning, drill string washout, junk in the hole, casing collapse, or excessive drag. Stuck pipe occurrences are widely held to be the most expensive drilling problem confronting the petroleum industry and the cost of correcting even a single occurrence can amount to millions of dollars. Most of the prior art regarding stuck pipe has been directed toward tile development of drilling techniques to minimize the risk of stuck pipe and to free stuck pipe once it has occurred. Statistics and statistical analysis have been used to improve the probability of freeing stuck pipe once sticking has occurred. The article "Stickiness Factor: A New Way of Looking at Stuck Pipe" by T. E. Love, published in the Oct. 3, 1983 issue of Oil & Gas Journal is one example. Until recently, however, very little effort has been directed to using statistical techniques for predicting the likelihood of stuck pipe under given conditions.
The most significant prior art in this area is disclosed in U.S. Pat. No. 4,791,998 issued to Chevron Research Co. as assignee of the inventors. The method disclosed in the '998 patent centers on multi-variate statistical analysis of "well drilling variable quantities" obtained from a number of previously drilled wells. These well drilling variables, or field variables, are measured quantities taken from observations of drilling conditions over the history of previous wells. These previous, or historical, wells are divided into classes according to the end result of the drilling, i.e., those wherein the drill string became mechanically stuck, those wherein the drill string became differentially stuck, and those wherein the drill string did not become stuck. The statistical analysis is performed on all of the measured values of the field variables of all the historical wells.
The results of the analysis are plotted to construct a "stuck pipe probability map". The map is then divided into three areas representing high likelihoods of the drill string remaining unstuck, becoming mechanically stuck, or becoming differentially stuck. Values corresponding to the field variables from the historical wells are then taken from a well being drilled, analyzed, and plotted on the map. The likelihood of the drill string becoming stuck can then be monitored by examining the position of the representation of drilling in the current well relative to the areas of high probability on the map. As the representation moves closer to areas of neutral or high probability of sticking, steps may be taken to improve drilling conditions by altering the values of selected variables to reduce the likelihood of sticking.
As is true of most methods for accomplishing a given task, the method disclosed in the '998 patent has been shown to be capable of improvement. The method disclosed and claimed herein is better in the modeling of probability and in applying the model to modify drilling conditions than the '998 patent's method some of whose field variables are highly interdependent, which results in two undesirable consequences. First, application of the model to current drilling conditions results in recommended actions contradictory to basic physics and/or logical engineering judgment to an unacceptably high degree. For example, two variables that have a high negative correlation would tend to cancel out if used simultaneously. Second, the effect of individual parameters on overall drilling conditions is difficult to ascertain and thereby impedes determination of corrective measures.
The '998 patent's method does not provide a quantitative assessment of the probability of sticking and assumes equal prior probabilities for each of the three groups. The method merely quantifies the probability that its indication of which result is more likely, i.e., whether it is more likely that the drill string will remain unstuck, become mechanically stuck, or become differentially stuck, is correct. Independent research has indicated in the meantime that the prior probabilities of falling into one of the three groups is far from being equal.
The '998 patent's method also indicates the most likely result only at the time the measurements are taken and does not provide a specific lag time in which drilling engineers may take corrective measures. The aggregate of these characteristics renders the '998 patent's method suitable primarily for surveillance. Drilling engineers need a more versatile tool that provides a quantitative assessment of the likelihood of sticking within a given time frame that also provides an indication of what corrective action to take so that they may evaluate the need for such corrective action and have time to implement it.
It is therefore a feature of this invention that it provides a more accurate method of modeling the likelihood that a particular drill string will become stuck.
It is furthermore an object of this invention to provide a method of modeling the likelihood of sticking that will yield a quantitative assessment of the probability.
It is still a further feature of this invention to provide a method of modeling the likelihood of sticking to provide drilling engineers with a better indication of which drilling variables need to be changed and how they should be changed in order to prevent stuck pipe.
It is further a feature of this invention to provide a method for ascertaining the probability of becoming stuck that will provide drilling engineers with a sufficient time period in which to evaluate the need for and take corrective action.
It is still further a feature of this invention to provide a method for applying an improved model for the probability of sticking to drilling conditions in a well being drilled to ascertain the quantitative probability of sticking within a given time frame, to compare this probability to the probability experienced in the past under similar conditions, and to determine the leading factors contributing to any overly high probability.